1. Field of the Invention
The present invention relates to a capacitive pressure sensor, more particularly to a capacitive pressure sensor that has an area that changes according to a variation of the external pressure so that the output changes linearly according to the input, and uses hafnium oxide, which has excellent insulating properties and a high dielectric constant, as an insulating material of a capacitor so as to have high sensitivity, and a method for fabricating said capacitive pressure sensor.
2. Description of the Related Art
Research in semiconductor pressure sensors using a micromachining technique employing a semiconductor fabricating process has been going on for quite some time. Semiconductor pressure sensors obtained by the micromachining technique are widely applied to vehicle systems, industrial control, environmental monitoring, and biomedical diagnosis fields.
Pressure sensors are elements for measuring an absolute pressure or a relative pressure, and are divided into strain gauge-type metal pressure sensors, piezoresistive pressure sensors, piezoelectric pressure sensors, MOSFET pressure sensors, piezojunction pressure sensors, optical fiber pressure sensors, and capacitive pressure sensors, according to sensing principles. The above pressure sensors, except for the metal pressure sensors, are fabricated by the micromachining technique using a semiconductor substance, i.e., silicon.
Capacitive pressure sensors use a principle in which the capacitance of a parallel plate capacitor between a silicon thin film diaphragm (membrane) and a support changes according to a variation of a gap between two electrodes due to the deflection of the silicon thin film diaphragm (i.e., the deformation of the membrane) according to a variation of pressure supplied from the outside.
The above capacitive pressure sensors are classified into absolute pressure sensors and relative pressure sensors. An absolute pressure sensor is a capacitive pressure sensor that is fabricated with an enclosed diaphragm, and thus senses an external pressure on the basis of a designated pressure in a reference pressure chamber. The absolute pressure sensor is completely isolated from the outside, thus it is not influenced by the surrounding environment. However, air in the reference pressure chamber is compressed by the deflection of the diaphragm, generating nonlinearity, and is expanded by an increase in temperature, generating undesirable offset and temperature drift. A relative pressure sensor is a capacitive pressure sensor that has a reference pressure open to the environment through a perforated glass portion, and thus senses the relative pressures of two portions, i.e., a difference of pressures between the upper and lower portions of a diaphragm. The relative pressure sensor is open to the outside, thus being easily influenced by surrounding substances. However, since the reference pressure chamber is not closed, the relative pressure sensor is not influenced by the compression or expansion of the air in the reference pressure chamber.
The capacitive pressure sensors, which detect a capacitance between two electrodes using the deflection of a diaphragm (the deformation of a membrane), have a sensitivity several hundred times as high as piezoresistive pressure sensors, and have higher stability (lower temperature coefficient and stronger structure) and lower power consumption than piezoresistive pressure sensors.
FIG. 1-1A to 1F illustrate a process for fabricating a conventional capacitive pressure S sensor. Since the variation in capacitance according to a variation in pressure is on the order of picofarads, in order to remove a parasitic capacitance between the two electrodes, a lower electrode 14 is formed on a glass substrate 13, an upper electrode 12 is formed on a silicon substrate 11, and the two substrates 13 and 11 are hybrid-bonded using an electric (anodic) bonding method.
First, FIG. 1-1A illustrates the silicon substrate 11 used as an upper substrate. As shown in FIG. 1-1B, the front surface of the silicon substrate 11 is etched to a designated depth by dry etching, and the upper electrode 12 of a capacitor, which is made of a metal, is formed on the etched region.
FIG. 1-1C illustrates the glass substrate 13 used as a lower substrate. As shown in FIG. 3-3D, the lower electrode 14 is formed on the glass substrate 13.
As shown in FIG. 1-1E, the two substrates 11 and 13 provided with the upper and lower electrodes 12 and 14 are hybrid-bonded using the electric (anodic) bonding method.
Finally, as shown in FIG. 1-1F, the rear surface of the silicon substrate 11 is etched by anisotropic chemical etching, thus forming a membrane. Thereby, the fabrication of a capacitive pressure sensor is completed.
The above capacitive pressure sensor electrically reads the change of distance between two electrodes of the capacitor through the movement of the membrane according to a variation of pressure.
The above conventional capacitive pressure sensor has excellent sensitivity and temperature characteristics (i.e., has higher sensitivity and is less sensitive to a variation of temperature) compared to a piezoresistive pressure sensor, thus being applicable to many fields. However, since the capacitance is in inverse proportion to the distance between two electrodes and the membrane does not maintain its planar shape while it moves downward but is bent when external pressure is applied thereto, the variation of capacitance according to a variation of pressure cannot exhibit linearity. That is, the linearity of output according to input is poor, and thus, it is difficult to achieve sensor compensation. Thereby, since the usable pressure range is narrow, the substantial use of the conventional capacitive pressure sensor is restricted.
Accordingly, in order to reduce the nonlinearity of the capacitive pressure sensor, various researches in the structural deformation of the capacitive pressure sensor has been carried out.
Various approaches for deforming the structure of the capacitive pressure sensor have been taken, as follows.
Korean Patent Publication No. 96-006113 discloses a semiconductor capacitive pressure sensor in which a capacitor electrode of a silicon diaphragm is deformed into a cross shape to improve nonlinear response characteristics. Further, Korean Patent Registration No. 10-0404904 discloses a differential capacitive pressure sensor and a method for fabricating the same, in which two sensing capacitors are disposed at both sides and a single-layered sacrificial layer is used to improve nonlinear characteristics of the pressure sensor. The intervals between the upper and lower electrodes of the two sensing capacitors become equal, using the single-layered sacrificial layer to simplify a fabricating process, and variations of capacitance of the sensing capacitors due to displacement become equal to increase the linearity of the pressure sensor.
See Aziz Ettouhami et al. (2004). “A novel capacitive pressure sensor structure with high sensitivity and quasi-linear response”. Morocco. Competes rendus. Mecanique. pp. 141-162.
In a general capacitive pressure sensor, when a membrane is thin in order to increase the sensitivity of the capacitive pressure sensor, the linearity of the capacitive pressure sensor is deteriorated. However, Aziz Ettouhami et al. solve the above nonlinearity problem using a series capacitor, thus improving the sensitivity and the linearity of a capacitive pressure sensor.
Further, see Satoshi Yamamoto et al. (2003). “Touch mode capacitive pressure sensor for passive tire monitoring system”. Japan. IEEJ Transactions on Sensors and Micro machines. pp. 9-15.
Satoshi Yamamoto et al. disclose a capacitive pressure sensor having high linearity, in which the distance between two electrodes of a capacitor is changed according to a variation of pressure. Such a capacitive pressure sensor has a principle in that, when pressure of a designated value or more is applied to the capacitive pressure sensor, the upper electrode, i.e. the membrane, contacts an insulating material deposited on the lower electrode.